Object surface be cracked when move too far grid center
This seems to be due to an old bug. The cause has been identified:


  The issue is actually coming from the way how ray bounce is happening: it is using ray_offset() in order to avoid self-intersection. This function will push ray along the face normal by a small value. This small value is different for different axis and actually depends on magnitude of the original coordinate. In this case ray is getting pushed a bit more along Y axis than along X and Z axis. This makes ray to be moved into the surface itself, causing light artifacts.


But it's apparently difficult to solve. There is a patch you can try if you are familiar with building software, but you'll likely have to apply it to an older version of blender (and it doesn't fix everything).